Step 4: sewing - Sandwich side 1
Align the edge of the inside circle with the outside circle. Sandwich the top side of the zipper-strip, facing the outside circle. Place needle at the top of the zipper head, where the zipper meets the strip. Make sure zipper is zipped.
Use a narrow or zipper foot, if you have one.
Chose an alignment on the plate of your sewing machine and use this measure consistently. I like working with the 1/4 inch mark.